ATM Data Protocol
The demand for fast, efficient and accurate transmission of digital data has vastly increased in recent years. In response to this demand, international standards for the transmission of digital data have been established and implemented. One such standard that is widely accepted and used is known as the Asynchronous Transfer Mode, or ATM.
ATM is a data transmission protocol that provides a standardized data format for the transmission of digital data over a high speed data transmission network and that facilitates the transmission of data belonging to numerous different applications on a single network. An important advantage of the ATM protocol is that it provides a single transmission format by which data from a variety of network data sources can be transmitted in a single transmission format, rather than requiring separate transmission formats and processing facilities for each type of data.
In the ATM protocol, all data is transmitted and received in the form of fixed-length packets or "cells," of 53 bytes each. Each 53 byte ATM cell includes a 48 byte payload and a 5 byte header. This header includes a virtual channel identifier (VCI) that indicates the particular channel or connection to which the cell belongs and is used to direct the cell to various switching points in the network. The ATM cell header also includes a one byte Header Error Check (HEC) that provides for 1 bit error correction and 2 bits error detection. Therefore, if a single bit in the 53 byte cell is in error, that bit can be corrected using the HEC, and if two bits are in error, the HEC will allow the receiving station to identify that a transmission error has occurred.
In an ATM communication system, the HEC is also used to delineate the ATM cells, which permits cell delineation without requiring any additional overhead bits to flame the ATM cell. Instead, the ATM cell framing is established by monitoring a received data stream for valid HEC bytes. If valid HEC bytes spaced exactly 53 bytes apart are detected for several consecutive flames, then valid ATM cell framing has been determined. Once this flaming has been determined, the receiving station can be synchronized to the incoming ATM data stream so that payload data can be received and processed.
This method of cell delineation, however, does not work well in high noise environments, such as radio transmission environments. Excessive bit errors can cause consecutive cells to register as having an error in the HEC byte, and thus cause the receiving end to assume the link is out of synchronization. The receiver at the receiving end will then go into a re-synchronization mode which, as described above, will search for a certain predetermined number of consecutive valid HEC bytes in order to re-establish link synchronization. During this re-synchronization process, however, data transmission will be suspended. Since payload data is not used during the re-synchronization process, ATM cells containing valid data can be lost. In a low data rate communications link, this data loss leads to additional delay and inefficiency in data transmission.
The ATM technique is referred to as being asynchronous because the slots in the frames of the signals are not reserved for the cells of particular applications, but instead are filled by the cells of various applications in accordance with the various applications' current demand for slots and the current availability of slots.
Each standardized ATM cell can be transmitted using any suitable data communications network. Typically ATM cells are transmitted over a standard telecommunications channel using a high speed network such as a Synchronized Optical Network (SONET), in which each ATM cell is provided to a slot in a frame of the SONET signal. Traditionally, the ATM standard has been used in the transmission of broadband telecommunications services such as B-ISDN and other high speed data communications applications.
Previously, however, there has not been recognized a need or ability to use the ATM data format for low speed data communications and particularly not for low speed radio data applications. The wide acceptance of the ATM standard for transferring data over standard telecommunications channels, however, facilitates fast, easy, worldwide data communications and therefore makes it an attractive option where standardized data formats are desired.
As used herein, the term "standard telecommunications channel" includes any data communication channel suitable for high speed digital data transmission, such as a Synchronized Optical Network, or SONET. Typically, such data communication channels are wireline based, but wireless transmission should not be excluded if similar data transmission performance can be achieved. Furthermore, such data communication channels generally experience lower bit error rates than comparable radio data transmission channels.
Radio Communications
Radio communications between a base station or headend and a plurality of mobile substations (such as aircraft or automobiles) have typically been accomplished through the use of push-to-talk (PTT) voice systems. More recently, data communication systems have been used. However, due to the noisy environment often found in radio systems, high speed data communication can be difficult. This is especially true where data communication is to occur between one or more mobile stations. As these mobile stations move, radio frequency propagation and interfering signals vary in magnitude and frequency. This can result in radical and rapid changes in data transmission errors.
One solution to this problem is to provide error correction data in the data transmission itself. This is known as forward error correction and allows for a number of data errors to be corrected by the receiving station. The use of forward error correction, however, results in a reduced data throughput when the transmission channel is of high quality since the forward error correction data is not needed to correct bit errors and is simply discarded by the receiving station. Another solution is to reduce the data rate of the data transmission, and to thereby reduce the number of data errors in a given time period. This of course may be unsatisfactory if substantial data throughput is required.
Regardless of the method used to ensure accurate data transmission over the radio communication link, compatibility problems can arise due to radio specific data protocols being mismatched with commonly used wireline data transmission standards. Particularly, if a complex error correcting protocol is used on a wireless radio link, while a simple checksum error detecting protocol is employed on a wireline link, then data format incompatibilities can result. To overcome these compatibility problems, the data must be converted from one format to the other. Such conversion can take considerable time and require substantial hardware or software to accomplish. These format incompatibilities also increase the difficulty in adapting a standard wireline telecommunications protocol, such as ATM, to a wireless environment.
Another problem in using ATM in the wireless environment is that ATM is normally used in the telecommunications environment, which is an isochronous network with clock stability synchronized with a known common network clock source. Commonly known as the Stratum clock hierarchy, this clock runs at 8 kHz, and provides the standard for all telecommunications channels with the basic rate of multiples of 64 kbits per second (kbps). This clock is critical because it provides timing information to extract voice, video and many other time "sensitive" services. The wireless environment on the other hand, runs synchronously or asynchronously without a network clock synchronization on a bit rate historically tied to teletypes or modems, which is generally an integer multiple of 300 bps. However, the 8 or 64 kbps data rates are not even multiples of the 300 bps data rate and therefore timing information required to carry video or voice over ATM must be conveyed somehow between the two system with sufficient accuracy to provide long term stability.